1. Field of the Invention
The invention relates to the field of electronic circuits, and in particular, to an efficient, low noise fractional charge pump.
2. Related Art
White LEDs (light emitting diodes) are becoming increasing common in modern electronic devices. For example, portable electronics such as cell phones, personal digital assistants (PDAs), and game systems often include graphical displays that are backlit by white LEDs. However, the battery power used in such devices generally provides a supply voltage that is below or just at the voltage required to drive white LEDs (typically around 3.6 V). For example, a modern rechargeable lithium ion or lithium polymer battery is typically rated to have a nominal output voltage of 3.7 V, but may actually provide a voltage in the range of 2.7 to 4.2 V, depending on the charge state of the battery.
This variability in battery supply voltage necessitates voltage boosting circuitry in most white LED-based portable devices. One of the most common schemes is to use a charge pump. A charge pump uses the charge storage capabilities of capacitors to generate an output voltage that is elevated with respect to a supply voltage. Modern white LED-based portable devices generally use one of two types of conventional charge pumps.
The first type of charge pump is known as a “2×” charge pump (sometimes referred to as a “voltage doubler”). FIGS. 1A and 1B show schematic diagrams of a conventional 2× charge pump 100 for receiving an input voltage V_IN1 and providing an elevated output voltage V_OUT1 to a load D130 (depicted as an LED for exemplary purposes). Charge pump 100 includes an input terminal 101, a charging capacitor C110, a storage capacitor C120, and an output terminal 102. While not shown for clarity, charge pump 100 also includes interconnect circuitry for connecting capacitors C110 and C120 in the configurations shown in FIGS. 1A and 1B.
Charge pump 100 operates by switching between the two phases of operation shown in FIGS. 1A and 1B. In FIG. 1A, a charging phase is shown, in which capacitor C110 is connected between input terminal 101 and ground, while capacitor C120 is connected between ground and output terminal 102 (load D130 is always connected between output terminal 102 and ground). During this charging phase, capacitor C110 is charged by input voltage V_IN1 to a voltage V11. Because capacitor C110 is connected directly between input voltage V_IN1 and ground, the voltage V11 stored on capacitor C110 will eventually be equal to input voltage V_IN1. During this charging of capacitor C110, a voltage V12 stored on capacitor C120 is provided as output voltage V_OUT1 for driving load D130.
Then, in a discharging phase shown in FIG. 1B, capacitor C110 is connected between input terminal 101 and output terminal 102, with capacitor C110 inverted with respect to input terminal 101. In other words, during the charging phase depicted in FIG. 1A, the positive plate in capacitor C110 (marked with a triangular indicator) is connected to input terminal 101, while the negative plate (unmarked) is connected to ground, thereby allowing input voltage V_IN1 to charge capacitor C110. Then, during the discharge phase depicted in FIG. 1B, the negative plate of capacitor C110 is connected to input terminal 101. The potential across capacitor C110 (i.e., voltage V11) is therefore added to input voltage V_IN1, thereby providing the sum of voltages V_IN1 and V11 at output terminal 102. Since voltage V11 is raised to voltage V_IN1 during the charging phase (FIG. 1A), the sum of voltages V_IN1 and V11 is simply twice the input voltage (i.e., 2*V_IN1). Thus, during the discharging phase (FIG. 1B), twice the input voltage V_IN1 is provided at output terminal 102 to drive load D130. At the same time, capacitor C120 is charged to twice the input voltage. When charge pump switches back to the charging phase, capacitor C120 resumes driving load D130 while capacitor C110 is recharged by input voltage V_IN1.
In this manner, 2× charge pump 100 can raise a battery voltage (e.g., 3.7 V) to a level (e.g., 7.4 V) that is more than sufficient to drive a white LED. However, the voltage doubling provided by 2× charge pump 100 is typically a much larger scaling effect than is required for white LED applications. Specifically, as noted above, a white LED typically exhibits a forward voltage in the range of 3.5 to 4.0 V, while a lithium ion battery source may provide anywhere in the range of 2.7 V to 4.2 V. Therefore, an LED driver circuit incorporating charge pump 100 would require attenuation circuitry to drop the 5.4 to 8.4 V charge pump output voltage down to the level required by the white LED. Such attenuation circuitry (e.g., a resistor in series with the LED) may dissipate a significant amount of power to provide the desired voltage attenuation, and can therefore substantially reduce the power efficiency of an LED driver circuit that incorporates 2× charge pump 100. Therefore, to improve the efficiency of charge pump-based LED driver circuits, a “3/2×” charge pump are commonly used.
FIGS. 2A and 2B show schematic diagrams of a conventional 3/2× charge pump 200 for receiving an input voltage V_IN2 and providing an elevated output voltage V_OUT2 to a load D240 (depicted as an LED for exemplary purposes). Charge pump 200 includes an input terminal 201, charging capacitors C210 and C220, a storage capacitor C230, and an output terminal 202. While not shown for clarity, charge pump 200 also includes interconnect circuitry for connecting capacitors C210, C220, and C230 in the configurations shown in FIGS. 2A and 2B.
Charge pump 200 operates by switching between the two phases of operation shown in FIGS. 2A and 2B. In FIG. 2A, a charging phase is shown, in which capacitors C210 and C220 are serially connected between input terminal 201 and ground, while capacitor C230 is connected between ground and output terminal 202 (load D240 is always connected between output terminal 202 and ground). During this charging phase, capacitors C210 and C220 are charged by input voltage V_IN2 to voltages V21 and V22. Under steady state conditions, capacitors C210 and C220 will both be charged to half of input voltage V_IN2 during this charging phase. Meanwhile, a voltage V23 stored on capacitor C230 is provided as output voltage V_OUT2 for driving load D240.
Then, in a discharging phase shown in FIG. 2B, capacitors C210 and C220 are connected in parallel between input terminal 201 and output terminal 202. In a manner similar to that described with respect to FIG. 1B, capacitors C210 and C220 are inverted with respect to input terminal 201. Specifically, whereas during the charging phase, the positive plate (marked with a triangular indicator) of capacitor C210 is connected to input terminal 201, during the discharging phase, the negative plate (unmarked) of capacitor C210 is connected to input terminal 201. Likewise, whereas during the charging phase, the positive plate (marked with a triangular indicator) is the upstream plate (i.e., the plate “closer” to input voltage V_IN2), during the discharging phase, the negative plate (unmarked) of capacitor C220 is connected to input terminal 201. Hence, during the discharging phase, the negative plate of capacitor C220 is the upstream plate.
Because capacitors C210 and C220 are inverted and connected in parallel after input terminal 201, the output voltage V_OUT2 provided during the discharging phase shown in FIG. 2B is equal to the sum of input voltage V_IN2 and the average of voltages V21 and V22 on capacitors C210 and C220, respectively. As described above with respect to FIG. 2A, both capacitors C210 and C220 are charged to half of input voltage V_IN2 during the charging phase. Therefore, the output voltage V_OUT2 provided during the discharging phase is simply equal to one and a half times input voltage V_IN2 (i.e., 1.5*V_IN2).
Therefore, the output voltage range of 3/2× charge pump 200 is between 4.05 V and 6.3 V when provided with a lithium ion battery voltage (i.e., 2.7 V to 4.2 V) as in input voltage. Because the output voltage range provided by 3/2× charge pump 200 is lower than that provided by 2× charge pump 100 for a given input voltage range, a white LED driver circuit incorporating 3/2× charge pump 200 requires less voltage attenuation than a driver circuit incorporating 2× charge pump 100, thereby allowing the 3/2× charge pump-based driver circuit to exhibit greater power efficiency.
However, as portable devices become increasingly advanced while at the same time shrinking in size, power efficiencies must continually be improved. While 3/2× charge pump 200 can reduce power consumption in a white LED driver circuit over 2× charge pump 100, 3/2× charge pump 200 is not ideally suited for such an application, since the forward voltage for a white LED (i.e., 3.5 to 4.0 V) is still significantly lower than even the lowest output voltage provided by the 3/2× charge pump 200 (i.e., 4.5 V). This voltage difference represents an inefficiency that will ultimately reduce overall device battery life.
Therefore, a 4/3× charge pump has been developed that applies a 4/3× multiplication factor to an input voltage (e.g., a 2.7 V to 4.2 V input voltage range is transformed into a 3.6 V to 5.6 V output voltage range). This output voltage range is still sufficient to drive white LEDs, but requires less voltage attenuation at the top end. Therefore, a 4/3× charge pump can improve LED drive circuit efficiency over 3/2× charge pumps and 2× charge pumps.
Conventional 4/3× charge pumps include three charging capacitors and one storage capacitor to provide the desired 4/3× voltage multiplication. FIGS. 3A and 3B show schematic diagrams of a conventional 4/3× charge pump 300 for receiving an input voltage V_IN3 and providing an elevated output voltage V_OUT3 to a load D350 (depicted as an LED for exemplary purposes). Charge pump 300 includes an input terminal 301, charging capacitors C310, C320, and C330, a storage capacitor C340, and an output terminal 302. While not shown for clarity, charge pump 300 also includes interconnect circuitry for connecting capacitors C310, C320, C330, and C340 in the configurations shown in FIGS. 3A and 3B.
Charge pump 300 operates by switching between the two phases of operation shown in FIGS. 3A and 3B. In FIG. 3A, a charging phase is shown, in which capacitors C310, C320, and C330 are serially connected between input terminal 301 and ground, while capacitor C340 is connected between ground and output terminal 302 (load D350 is always connected between output terminal 302 and ground). During this charging phase, capacitors C310, C320, and C330 are charged by input voltage V_IN3 to voltages V31, V32, and V33, respectively. Under steady state conditions, capacitors C310, C320, and C330 will be charged to one third of input voltage V_IN3 during the charging phase. Meanwhile, a voltage V34 stored on capacitor C340 is provided as output voltage V_OUT3 for driving load D350.
Then, in a discharging phase shown in FIG. 3B, capacitors C310, C320, and C330 are connected in parallel between input terminal 301 and output terminal 302. In a manner similar to that described with respect to FIG. 2B, capacitors C310, C320, and C330 are inverted with respect to input terminal 301. Therefore, the output voltage V_OUT3 provided during the discharging phase shown in FIG. 3B is equal to the sum of input voltage V_IN3 and the voltage on each of capacitors C310, C320, and C330. As described above with respect to FIG. 3A, C310, C320, and C330 are all charged to one third of input voltage V_IN3 during the charging phase. Therefore, the output voltage V_OUT3 provided during the discharging phase is simply equal to one and one third times input voltage V_IN3 (i.e., 4/3*V_IN3).
Therefore, the output voltage range of 4/3× charge pump 300 is between 3.6 V and 5.6 V when provided with a lithium ion battery voltage (i.e., 2.7 V to 4.2 V) as in input voltage. Because the output voltage range provided by 4/3× charge pump 300 is lower than that provided by 3/2× charge pump 200 for a given input voltage range, a white LED driver circuit incorporating 4/3× charge pump 300 requires less voltage attenuation than a driver circuit incorporating 3/2× charge pump 200, thereby allowing the 4/3× charge pump-based driver circuit to exhibit greater power efficiency.
Unfortunately, this improved power efficiency provided by 4/3× charge pump 300 comes at the expense of an additional capacitor (C330) over 3/2 charge pump 200. The relatively large capacitance values (and hence, capacitor sizes) required to provide reasonable charge pump power capabilities can preclude inclusion of the additional capacitor “on chip”, due to die size limitations. Moving the capacitor(s) “off chip” reduces this size burden but adds a new requirement of additional pins in the chip package to communicate with the external capacitor(s). As a result, chip package size and cost is negatively impacted.
Accordingly, it is desirable to provide a system and method for driving white LEDs that maximizes power efficiency while minimizing die area requirements.